Zeolite materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of large dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as rigid three-dimensional frameworks of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This balanced electrovalence can be expressed by a formula wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. These zeolites have come to be designated by zeolite A (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,970); and zeolite ZSM-12 (U.S. Pat. No. 3,832,449), merely to name a few.
Although the term, zeolites, encompasses materials containing silica and alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. More particularly, GeO.sub.2 is an art recognized substitute for SiO.sub.2 Also, B.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, and Ga.sub.2 O.sub.3 are art recognized replacements for Al.sub.2 O.sub.3. Accordingly, the term zeolite as used herein shall connote not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and/or aluminum. On the other hand, the term aluminosilicate zeolite as used herein shall define zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof, as opposed to materials which contain substantial amounts of suitable replacement atoms for such silicon and/or aluminum.
Although certain zeolites can be prepared from totally inorganic reaction mixtures, the synthesis of other zeolites is often promoted or made possible by the inclusion of certain organic compounds, termed organic directing agents, in the reaction mixture. Note the article by Lok et al., entitled "The Role of Organic Molecules in Molecular Sieve Synthesis" appearing in Zeolites, Vol. 3, pp. 282-291 October, (1983). When such organic directing agents are used, they may be included in an aqueous reaction mixture containing reactants, e.g., sources of silica and alumina, necessary for the zeolite synthesis.
The reaction mixture may then be maintained under sufficient conditions, e.g., at elevated temperature, until the desired crystals are formed. These crystals may then be recovered by filtration and washing the filtered crystals with water. This filtering and washing treatment separates the crystals from organic directing agent which is either included in the mother liquor of the reaction mixture or loosely associated with the exterior surface of the crystals. However, a residue of the organic directing agent, e.g., amines and especially quaternary ammonium compounds, usually remains more tenaciously attached to the zeolite crystals.
This tenaciously attached residue, which is not removed by the filtering and washing treatment, may be occluded within the pores of the zeolite and/or firmly affixed to the surface of the zeolite. Certain residues which are tenaciously attached to the zeolite may occupy cation exchange sites of the zeolite, especially in the case of quaternary ammonium residues It is particularly important to remove organic residue which occluded in the pores of the zeolite, because this type of residue may constitute obstructions which tend to substantially reduce the sorption capacity and catalytic activity of the zeolite.
In order to remove the residue of organic directing agents from as-synthesized zeolites, which residue cannot be readily removed by filtration and washing, the zeolite may be calcined at elevated temperatures, e.g., 400.degree. C. or greater, e.g., in the presence of a source of oxygen such as air, for, e.g., at least one hour. This calcination treatment promotes the decomposition and/or volatilization of the residue. The presence of oxygen during the calcination further promotes oxidation, e.g., combustion, of the organic residue into oxidized species, e.g., carbon dioxide, carbon monoxide, water, and nitrogen oxides, which are evolved as gasses.
The above-mentioned calcination procedure is effective for removing organic directing agent residue from as-synthesized zeolites which are stable under the required conditions. However, certain as-synthesized zeolites tend to undergo a phase transformation, e.g., to a different crystalline form or to an amorphous material, under these conditions. One such zeolite is ZSM-18, especially species of ZSM-18 which have a low silica to alumina molar ratio. U.S. Pat. No. 3,950,496 issued to Ciric, the entire disclosure of which is expressly incorporated herein by reference, describes ZSM-18 and the synthesis thereof. Example 6 of this Ciric patent points out that when the as-synthesized form of ZSM-18, prepared from a reaction mixture having a silica to alumina molar ratio of 9.0, was calcined at 1,000.degree. F. in air for 3 hours, the zeolite was substantially reduced to the amorphous state.
Ryan in U.S. Pat. No. 4,851,200 which issued on Jul. 25, 1989, disclosed a three-step method for removing a residue of an organic directing agent from as-synthesized ZSM-18 zeolites. The first step involved contacting as-synthesized zeolite with an aqueous solution of an ammonium fluorosilicate. In the second step, the zeolite is ion-exchanged with an alkali metal or alkaline earth metal salt. During the third step, the ion-exchanged zeolite is air calcined while co-feeding ammonia. These steps are repeated at least once.
Therefore, what is needed is a simple method to remove an organic residue from ZSM-18 which does not require a repetition of the process steps and where a noxious ammonia gas is not co-fed during air calcination.